This invention concerns aqueous drilling fluids containing anionic polymers. More particularly, this invention relates to a method of increasing the viscosity of aqueous drilling fluids using anionic dispersion polymers, to aqueous drilling fluids containing anionic dispersion polymers, and to use of the drilling fluids for drilling a well bore through a subterranean formation.
In drilling wells for the recovery of oil, gas and water from subterranean formations, a drilling fluid or drilling mud is pumped down through a hollow drill pipe, across the face of the drill bit, and upward through the drill hole. The drilling mud serves to cool and lubricate the drill bit, to raise the drilling cutting to the surface and to seal the sides of the well to prevent loss of water and drilling fluid into the formation surrounding the drill hole. The drilling mud must have both proper viscosity and some degree of gelation to carry the drilled solids to the surface, over a screen to remove the large chips, and to remove sands in a settling basin.
In order to achieve the proper viscosity, various polymeric viscosifying agents may be added to the drilling fluid. The polymers act to increase the viscosity of the drilling fluid, thus forming a barrier between the rock formations and the vein of material being recovered. Polymers also help to increase the slipperiness of the drilling fluid and drilled ore and to lubricate the waste sludge. Polymers are also useful in the separation of the water, oil and waste solids after the drilling fluid is recovered from the well.
Polymeric viscosifiers used to date include latex polymers, where polymer solids are dispersed in a hydrocarbon solvent and stabilized with surfactants, dry polymers and solution polymers.
Disadvantages associated with latex polymers include the environmental unfriendliness of hydrocarbons and surfactants in case of spill or discharge on land or on an off shore platform and the potential fire hazards associated with the hydrocarbon solvent. Furthermore, environmental regulations in certain countries prohibit the injection of oil-containing polymers into subterranean formations below a specified depth. Latex polymers also must be inverted prior to use and the oil in water emulsion broken, which involves the use of additional surfactants.
Dry polymers are conventionally used drilling operations due to the high polymer concentration available in this form as compared to latex or solution polymers. However, dry polymers are typically very difficult to dissolve, requiring significant energy and water consumption to assure adequate makedown of the dry polymer into an active dilute form. Also, because of the difficulty associated with dissolution of the dry polymer, only very dilute solutions, typically less than 0.5 percent polymer product are prepared, thus significantly increasing water use requirements. In remote drilling locations both energy and water are often in short supply and require significant financial input to secure. Accordingly, there is an ongoing need for the development of environmentally friendly, safe and economical viscosity modifiers for drilling fluids.
In an aspect, this invention is a method of increasing the viscosity of an aqueous drilling fluid used in drilling wells through subterranean formations comprising adding to the drilling fluid an effective viscosity increasing amount of one or more anionic dispersion polymers, wherein the anionic dispersion polymers are composed of about 2 to about 98 mole percent of one or more anionic monomers and about 98 to about 2 mole percent of one or more nonionic monomers and wherein the anionic dispersion polymer has a RSV of about 10 to about 50 dL/g.
In another aspect, this invention is an aqueous drilling fluid used in drilling wells through subterranean formations having a pH of between about 7 and about 12 and a viscosity of from about 20 to about 80 sec (Marsch cone) which is prepared by admixing fresh water, brine or seawater with about 0.02 lb to about 2.5 lb per barrel, based on polymer actives, of one or more anionic dispersion polymers, wherein the anionic dispersion polymers are composed of about 2 to about 98 mole percent of one or more anionic monomers and from about 98 to about 2 mole percent of one or more nonionic monomers and wherein the anionic dispersion polymers have a RSV of from about 10 to about 50 dL/g.
In another aspect, this invention is a method of drilling a well bore through a subterranean formation comprising circulating through the well bore an aqueous drilling fluid having a pH of between about 7 and about 12 and a viscosity of from about 20 to about 80 sec (Marsch cone) which is prepared by admixing fresh water, brine or seawater with about 0.02 lb to about 2.5 lb per barrel, based on polymer actives, of one or more anionic dispersion polymers, wherein the anionic dispersion polymers are composed of about 2 to about 98 mole percent of one or more anionic monomers and from about 98 to about 2 mole percent of one or more nonionic monomers and wherein the anionic dispersion polymers have a RSV of from about 10 to about 50 dL/g.
The drilling fluid of this invention is prepared using water-based polymer compositions that contain neither organic solvents or surfactants, thereby resulting in a treatment system that addresses the environmental, safety and handling and economic concerns of industries utilizing such drilling processes.
Drilling fluids prepared using the anionic dispersion polymer of this invention do not flocculate the bentonite in bentonite-containing drilling muds and therefore have significantly increased useable life compared to bentonite-containing drilling fluids prepared from dry polymers of similar composition.
Also, when the anionic dispersion polymer of this invention is used to prepare bentonite-containing drilling muds, less bentonite is required to prepare a drilling mud having the same lifting characteristics as the corresponding composition prepared using dry polymers, resulting in increased drill bit life. This allows the end-user to realize cost savings relating to bentonite as well as the costs associated with less-frequent replacement of the drill bits.